If you can no longer get macOS updates through App Store, you will need to manually download and install them from here: https://theapplewiki.com/wiki/Firmware/Mac/10.13.x [EDIT: they do not have the links to the latest available updates]
The links on the website are direct links to Apple servers
As for iTunes, you can download from here: https://theapplewiki.com/wiki/ITunes
According to https://en.wikipedia.org/wiki/History_of_iTunes the last compatible version for macOS 10.13 is iTunes 12.8.3
I don't know what happened but a few months ago, testing older macOS versions, only High Sierra is unable to check for updates... Could it be that Apple did something in the backend? Maybe...
Thanks, that's interesting you couldn't get the updates either. When updating Sierra to HS, there were no updates in the App Store for anything at all. Thankfully, bogdanw rescued me from the locked out iTunes, luckily which was the main concern for phone and back ups. iMovie is another story, but even QuickTime is still ancient and didn't update with HS. Maybe it's their way of telling us that we shouldn't be stuck on the old OS. But ya know, iPhone, iPad, and the MBP is last to get a look in when you're skint, needs must and all that... thank you bothBased on this thread https://forums.macrumors.com/threads/high-sierra-10-13-6-build-17g66.2169955/
17G66 is 10.13.6 without any of the later security updates.
Based on https://osxdaily.com/2020/11/19/sec...high-sierra-new-macos-11-0-1-build-available/
“Installing security updates with package installers is basically the same as using a combo update for MacOS updates.”
You should install just the last one: Security Update 2020-006 (High Sierra) https://support.apple.com/kb/DL2059
Maybe Apple cleaned up the update package from unnecessary files.